US

Hoover Dam, a concrete arch-gravity dam in the Black Canyon of the Colorado River on the border between the US states of Arizona and Nevada. Hoover Dam ,finished in 1936, impounds Lake Mead, the largest reservoir in the United States by volume. The dam's generators provide power for public and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist attraction; nearly a million people tour the dam each year.

Joe Klamar/AFP

The sun rises over the Hoover Dam, a concrete arch-gravity dam in the Black Canyon of the Colorado River on the border between the US states of Arizona and Nevada. The Hoover Dam was finished in 1936 and impounds Lake Mead, the largest reservoir in theUS by volume.

US soldiers carry a flag-draped coffin containing the remains believed to be of a US soldier during a repatriation ceremony at Phnom Penh International Airport.  Remains believed to be those of three US servicemen killed in Cambodia more than four decades ago during the Vietnam War were repatriated for identification in the United States

Siv Channa/AFP/Getty Images

US soldiers carry a flag-draped coffin containing the remains believed to be of a US soldier during a repatriation ceremony at Phnom Penh International Airport. Remains thought to be those of three US servicemen killed in Cambodia more than four decades ago during the Vietnam war were repatriated for identification in the US.

Largest Freefall Formation world record attempt in Arizona USA...epaselect epa04150210 222 skydivers from World Team attempt to break the world record for the Federation Aeronautique Internationale (FAI) Largest Sequential Freefall Formation attempt at Skydive Arizona in Eloy, Arizona, USA, 01 April, 2014.

Roy Dabner/EPA

222 skydivers from World Team attempt to break the world record for the Federation Aeronautique Internationale (FAI) Largest Sequential Freefall Formation attempt at Skydive Arizona in Eloy, Arizona, US on Monday. Read more

Obama Welcomes World Series Champions Boston Red Sox To The White House...WASHINGTON, DC - APRIL 01:  Boston Red Sox designated hitter David Ortiz (L) poses for a "selfie" with U.S. President Barack Obama during a ceremony on the South Lawn of the White House to honor the 2013 World Series Champion Boston Red Sox April 1, 2014 in Washington, DC. The Red Sox defeated the St. Louis Cardinals in the 2013 World Series. (Photo by Win McNamee/Getty Images)

Win McNamee/Getty

Boston Red Sox designated hitter David Ortiz poses for a selfie with US President Barack Obama during a ceremony at the White House to honour the 2013 World Series Champions Read more

Pope Francis and President Barack Obama smile as they exchange gifts, at the Vatican Thursday, March 27, 2014. President Barack Obama called himself a "great admirer" of Pope Francis as he sat down at the Vatican Thursday with the pontiff he considers a kindred spirit on issues of economic inequality. Their historic first meeting comes as Obama's administration and the church remain deeply split on issues of abortion and contraception. (AP Photo/Gabriel Bouys,

Gabriel Bouys/AP

Pope Francis and US President Barack Obama exchange gifts at the Vatican on Thursday. It is their first meeting.

Pope Francis Meets President Obama...VATICAN CITY, VATICAN - MARCH 27:  The silhouette of US President Barack Obama is seen as he arrives in his car at the San Damaso courtyard for a meeting with Pope Francis on March 27, 2014 in Vatican City, Vatican. This is Obama's first audience with Pope Francis and it is the second time that he has been received at the Vatican, after an audience with Pope Benedict XVI on July 10, 2009. Barack Obama is the ninth US President to make an official visit to the Vatican. The first, Woodrow Wilson, received by Pope Benedict XV after the end of the First World War.  (Photo by Franco Origlia/Getty Images)

Franco Origlia/Getty

US President Barack Obama arrives at the San Damaso courtyard for a meeting with Pope Francis in Vatican City, Vatican. This is Mr Obama’s first audience with Pope Francis, and it is the second time he has been received at the Vatican, after an audience with Pope Benedict XVI on July 10 2009. He is the ninth US president to make an official visit to the Vatican. The first, Woodrow Wilson, was received by Pope Benedict XV after the end of World War I.

World Leaders Gather For Nuclear Security Summit 2014...AMSTERDAM, NETHERLANDS - MARCH 24: US President Barack Obama (L) and Prime Minister of the Netherlands Mark Rutte (C) visit the Rijksmuseum ahead of the 2014 Nuclear Security Summit on March 24, 2014 in Amsterdam, Netherlands. The Nuclear Security Summit, held March 24-25, will be attended by world leaders and is aimed at preventing nuclear terrorism.

Jerry Lampen/Getty

Barack Obama, US president (left), and Mark Rutte, Netherlands prime minister (centre), meet in front of Rembrandt’s ‘The Night Watch’ in the Rijksmuseum in Amsterdam before the start of the 2014 Nuclear Security Summit. The summit, held March 24-25, will be attended by world leaders and is aimed at preventing nuclear terrorism.

A Filipino student shouts anti-US slogans outside the US embassy in Manila, Philippines, 21 March 2014 in a rally condemning the docking of the USS Blue Ridge and the upcoming visit of US President Barack Obama to the country. Activists also called for an end to the presence of US troops in the country and the Visiting Forces Agreement (VFA). Obama is to visit Japan, South Korea, Malaysia and the Philippines in late April. The trip is part of his efforts to broaden US engagement in the Asia-Pacific region on all fronts, from strategic security issues to trade and diplomacy. Obama had originally planned an Asia trip for late 2013, but was forced to cancel during the budget and debt limit crisis that closed the US government for more than two weeks in October.

Dennis M. Sabangan/EPA

A Filipino student protests outside the US embassy in Manila, Philippines, in a rally condemning the docking of the USS Blue Ridge and the upcoming visit of US President Barack Obama. Activists also called for an end to the presence of US troops in the country and the Visiting Forces Agreement.

This image provided by NASA March 16, 2014 shows a portion of the Florida Keys taken by Astronaut Rick Mastracchio from the International Space Station. (AP Photo/NASA, Rich Mastracchio)

Rich Mastracchio/Nasa/AP

This image provided by NASA shows a portion of the Florida Keys taken by Astronaut Rick Mastracchio from the International Space Station Read more

Muppet Kermit the Frog rings the opening bell at the New York Stock Exchange...Muppet Kermit the Frog rings the opening bell at the New York Stock Exchange March 17, 2014.  Kermit rang the bell to celebrate Disney's new movie "Muppets Most Wanted." REUTERS/Brendan McDermid (UNITED STATES - Tags: ENTERTAINMENT)

Brendan McDermid/Reuters

Kermit the Frog rings the opening bell at the New York Stock Exchange on Monday. Kermit rang the bell to celebrate Disney’s new movie Muppets Most WantedRead more